<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <script>
        //函数如果有return 则返回的是return 后面的值 如果没有return 则付汇undefined
        function fun1() {
            return 666;
        }

        console.log(fun1());//返回666
        function fun2() {

        }

        console.log(fun2());//返回undefined;
    </script>
</head>
<body>

</body>
</html>